The present study was undertaken to assess GH secretory profiles in 12 light-breed foals and their dams during forty days after delivery, and the possible influence of GRF and TRH on plasma GH concentrations in these newborn foals. GH secretory pattern was pulsatile in one day- as well as in forty days-old foals. The number of secretory spikes (10 per 24 h) did not vary between days 1 and 40. In the same times, GH secretion did not show any circadian rhythm either in foals or in their dams. Mean daily plasma concentrations (measured through blood samples collected every 20 min for 24 h) were lower in mares (3.4 +/- 0.3 ng/ml) than in their foals (7.4 +/- 0.9 ng/ml; P < 0.05). This difference resulted from both a lower number of GH spikes per 24 h (5 +/- 2 vs 10 +/- 1; P < 0.01) and from a lower pulse amplitude average (8 +/- 5 vs 16 +/- 1; P < 0.05). In three days- and in six days-old foals, synthetic human GRF (0.3 microgram/kg body wt, i.v.) significantly increased plasma GH concentrations. TRH (3 micrograms/kg body wt, i.v.) did not significantly modify plasma GH.